1. Field of the Invention
The present invention relates to controlled self-ignition four-stroke internal-combustion engines.
2. Description of the Prior Art
Controlled self-ignition is a well-known phenomenon in two-stroke engines. This combustion type has advantages as regards emissions: low hydrocarbon and nitrogen oxide emissions are notably obtained. Furthermore, a remarkable cycle regularity is achieved during self-ignition combustion.
Self-ignition is a phenomenon that allows to initiation of combustion by means of residual burned gases which remain in the combustion chamber after combustion.
Self-ignition is achieved by controlling the amount of residual gases and the mixing thereof with the fresh gases (unburned). The residual gases (hot burned gases) initiate the combustion of the fresh gases thanks to a combination of temperature and of presence of active species (radicals).
In two-stroke engines, the presence of residual gases is  less than  less than inherent greater than  greater than  in the combustion. In fact, when the load of the engine decreases, the amount of fresh gases decreases, which leads to an increase in the amount of residual gases (burned gases from the previous cycle or cycles which have not flowed out of the cylinder). The two-stroke engine thus works with an internal recirculation (or internal EGR) of the burned gases at partial load. However, the presence of this internal EGR is not sufficient to obtain the desired self-ignition running. Research work also shows that mixing of this internal EGR and of the fresh gases has to be controlled and limited.
The controlled self-ignition technology applied to four-stroke engines is particularly interesting because it allows the engine to run with an extremely diluted mixture, with very low fuel/air ratios and ultra-low NOx emissions.
However, this technology comes up against a significant technological difficulty insofar as, in order to obtain it without the internal EGR effect of the two-stroke engine, it is necessary to either significantly increase the compression ratio of the engine (with knocking problems at high load), or to considerably heat up the fresh gases admitted (several hundred degrees C), or to combine these two phenomena.
Solutions allowing decreasing pressure and temperature level requirements for four-stroke engines can be partly provided by suitable additives in the fuel. French patent 2,738,594 illustrates a solution of this type.
It is well-known for four-stroke engines, for example from international patent application PCT WO-93/16,276, to combine a variable distribution adjustment with a non-return system at the intake in order to reduce pumping losses at partial load. This solution then allows operation with the intake throttle as wide open as possible.
French patent application EN.97/02,822 of the assignee describes a way of controlling self-ignition in a four-stroke engine. More precisely, this document recommends, at partial load, to minimize mixing of the fresh gases and of the burned gases trapped in the combustion chamber by delaying closing of the exhaust as much as possible. This is an  less than  less than internal greater than  greater than  recycle that allows stratification of the gases in the combustion chamber.
Patent application EN.97/11,279 of the assignee also aims to minimize, at partial load, mixing of the fresh gases and of the burned gases contained in the combustion chamber, in order to control and to favour self-ignition combustion. However, this prior art proposes transferring the burned gases from the exhaust to the intake via a specific line opening into an air supply line just upstream from the combustion chamber. The fresh air-fuel feed is introduced separately and late via a second line. A successive introduction of the feeds is thus provided.
However, this solution creates a substantial dilution of the recycled burned gases, by air, prior to entering the combustion chamber, which may pose problems.
The present invention provides very simple and reliable controlled self-ignition in mullticylinder four-stroke engines, which is easy to implement and best favours stratification of the burned gases in the combustion chamber. Furthermore, the burned gases retain, according to the invention, their temperature, which is favorable to spontaneous combustion.
The present invention thus provides a controlled self-ignition combustion process in a four-stroke internal-combustion engine comprising several cylinders having each at least a first intake and at least a first exhaust.
According to the invention, the process consists, during part-load operation, in transferring, via at least one specific port of each cylinder and a suitable exhaust gas transfer, exhaust gases from a cylinder in the exhaust phase to another -cylinder in the intake phase.
The process according to the invention further controls the distribution of the flow of exhaust gases between the first exhaust and the exhaust gas transfer.
Furthermore, the process can thermally insulate and/or heat the exhaust gases transferred in the exhaust gas transfer, in order to improve self-ignition even further.
Furthermore, at partial load, an additional port is opened at a time close to the opening of the first exhaust means and a port is closed at a time close to the closing of the first intake.
According to an embodiment of the invention, a common line is used for the exhaust gas transfer.
More particularly, at full and high loads, the common line and at least some of the additional ports are used to introduce a feed or to discharge exhaust gases.
According to another embodiment of the invention, a set of lines connecting the cylinders in groups of two is used for exhaust gas transfer from cylinder to cylinder.
The present invention also relates to a four-stroke internal-combustion engine working on the controlled self-ignition principle and comprising several cylinders having each at least a first intake and at least a first exhaust.
According to the invention, each cylinder further comprises at least one additional port allowing passage of the exhaust gases from a cylinder in the exhaust phase to another cylinder in the intake phase, as well as an associated exhaust gas transfer, during part-load operation.
A device providing thermal insulation and/or heating of the exhaust gas can also be provided without departing from the scope of the invention.
The engine also advantageously comprises an exhaust gas distributor which distributes the exhaust gases between the first exhaust and the exhaust gas transfer additional ports, at partial load.
The exhaust gas distributor can comprise either a first throttling device placed close to the first exhaust, or a second throttling device placed near the exhaust gas transfer, or both throttling devices.
According to an embodiment, the exhaust gas comprises a common line opening into each additional port.
The common line advantageously also comprises a port into which a line provided with a gas flow control opens.
According to another embodiment of the invention, the exhaust gas transfer means comprises a set of lines connecting the cylinders in groups of two.